This invention relates to a method for improving the sound reproduced by audio systems in vehicles and, in particular, to a system which compensates for noise outside the audio system. When music or speech is reproduced in a vehicle, the signal is corrupted by external acoustic noise present in the vehicle. This noise may result from and is dependent upon vehicle speed, road condition, weather and condition of the vehicle. The presence of such noise results in a situation where soft sounds of interest are hidden, the perceived loudness is reduced and the intelligibility of the signal is lessened.
The vehicle's driver and/or passengers may compensate for increased external noise by increasing the volume of the audio system. However, when the vehicle speed decreases or another source of external noise is alleviated, the audio system's volume will be too high, requiring the user to reduce it. A frequent need to increase and reduce the volume is an inconvenience and can also be a safety hazard because it distracts the driver's attention. Thus, a system to automatically compensate for external noise by raising and lowering audio system volume makes driving both more convenient and safer.
Prior attempts to eliminate or reduce the effects of noise on sound signals reproduced in moving vehicles include a scheme known as "automatic level control," which automatically raises the volume when the vehicle is noisy. There are two major problems with such prior art systems.
First, these systems seldom determine the noise level properly because they use inaccurate means of estimating or measuring the noise level. Some automatic level control systems known in the prior art predict the noise level based on vehicle data such as speed, window status, gear ratio, etc. Such a measurement method may lead to incorrect noise estimates since many conditions, such as vehicle age, tire selection, and weather conditions, are overlooked.
Other prior automatic level control systems measure the noise level outside the vehicle. This method can lead to gross errors in the noise estimate, depending on the condition of the soundproofing of the car and other conditions, such as whether the windows are open. Other automatic level control systems known in the art measure the sound level inside the vehicle, which is the sum of the music and noise level. If the predicted music level is subtracted from the total noise level, an approximation of the noise level can be made. This technique requires careful calibration. Errors can cause a problem called "gain runaway," where increases in the music level are misconstrued as increases in the noise level, thereby leading to further increases in the music level.
The second major problem with automatic level control systems known in the prior art is that they do not provide a means to measure the music level. As will be shown herein, it is critical that any such system have complete and accurate measurements of both noise level and music level in order to accurately predict the impairment caused by the noise.
Systems known in the prior art to compensate for external noise use compression techniques to reduce the dynamic range of the reproduced signals. Such systems are also insufficient, as they are unable to differentiate between the music and the noise present in the vehicle. Sound quality is noticeably impaired since the music signal is audibly compressed when the vehicle is noisy. Such a compressed music signal does not sound the same as a non-compressed signal played in a quiet car.
When noise is present, reproduced music (or any other audio signal) appears to be lower in loudness than it actually is, The extent of loudness change was measured by Hellman and Zwislocki in Loudness Function of a 1000-cps Tone in the Presence of a Masking Noise. Journal of the Acoustical Society of America, pp. 1618-27, vol. 36, No. 9, September 1964 and is shown in FIG. 18. Such changes are also discussed in Handbook of Perception, ed. Canteetta and Friedman, Vol. 4 (1978). The amount of signal level reduction is a function of the signal to noise (S/N) ratio. If the signal level is raised an amount equal to the perceived level shift caused by the noise, the correction will be transparent to the listener, and the reproduced sound will appear to be unchanged by the noise.
Implementations known in the prior art for compensating for such noise have all used analog detection techniques which do not differentiate between the music and the noise. Thus, the masking effect of the noise was incorrectly estimated. In contrast, the present invention uses digital filtering techniques which measure the true noise level in the vehicle.